CONCRETE
LIBRARY
INFLUENCE
(Translation
Takao UEDA
OF JSCE NO. 35, JUNE
2000
OF DESALINATION ON MECHANICAL BEHAVIOR OF
PRESTRESSED CONCRETE MEMBERS
from Proceedings
of JSCE, No.613/V-42,
Masanobu ASHIDA
Shigeru
February
MIZOGUCHI
1999)
Toyo MIYAGAWA
The premature deterioration
of concrete structures due to chloride-induced
corrosion of
reinforcement has recently become a serious problem. Desalination
aims to remove chlorides
from concrete structures and is expected to enter service as a radical repair method. Desalination
has found actual to use with RC structures, but application to PC structures has been considered
difficult
because of the hydrogen embrittlement problem. This paper describes the influence of
desalination
on the behavior of both prestressing steel bars and PC beams.
Keywords: desalination, electrochemical technique, hydrogen embrittlement,
slow strain rate tensile test, hydrogen absorption, flexural test of beam
Takao Ueda is an assistant professor in the Department of Civil Engineering, Tokushima University,
Tokushima, Japan. He received his doctorate in engineering from Kyoto University in 1999. His
current research interests relate to electrochemical
repair methods for deteriorated
concrete
structures.
He is a member of the JSMS, JCI, and JSCE.
Masanobu Ashida is a manager in the technical
division
of the Special Cement Additive
Department at Denki Kagaku Kogyo Kabushiki Kaisha Co., Ltd., Japan. His research interests
relate to the technology of rehabilitating
concrete structures.
He is a member of the JSCE.
Shigeru Mizoguchi is a manager in the Engineering Sales Section of the Steels and Wire Products
Division at Neturen Co., JJ;d. His work is related to research and development on prestressing
steels, systems, and wire products.
He is a member of the JSCE.
Toyo Miyagawa is a professor in the Department of Civil Engineering, Kyoto University, Kyoto,
Japan. He received his doctorate in engineering from Kyoto University in 1985.
He is the author
of a number of papers dealing with the durability,
maintenance, and repair of reinforced concrete
structures.
He is a member of the ACI, RILEM, CEB, JSMS, JCI, and JSCE.
-53-
1.
INTRODUCTION
It has long been considered that, with adequate design and construction, reinforced concrete
structures were extremely durable. However, the early age deterioration of such structures caused
by chloride attack has recently become a serious problem.
Desalination
is an electrochemical
repair method that amis to remove chloride ions (CF) from concrete structures.
Desalination
makes use of currents that are approximately two orders of magnitude higher than those used for
cathodic protection, which is the most popular electrochemical
method at present. However,
desalination
has an advantage over other methods in that it is only temporary, unlike cathodic
protection.
Examples of the application
of desalination
to RC structures has been gradually increasing.
Recently, the repair effect has been confirmed [1] although some problems are indicated:
for
example, durability
after finishing desalination
[2].
On the other hand, there have been few cases
of desalination
being applied to PC structures because of the problem of hydrogen embrittlement of
the prestressing steel.
In this research, the influence of desalination
on hydrogen embrittlement of prestressing
steel and
on the behavior of PC beam members is investigated
by applying desalination
to pretension-type
PC specimens contaminated with chlorides.
2.
MECHANISM OF HYDROGEN EMBRTTTLEMENT
It is assumed that the following cathodic
cathode when desalination is applied:
2H2O +O2 +0e~
H2O+e~
reactions occur around the prestressing
-> 0OH-
steel, which acts as
(1)
->H+OH~
(2)
When adequate oxygen exists around the cathode and the cathodic potential
is nobler than the
hydrogen generation potential, reaction (1) becomes dominant. However, when these conditions
are not satisfied, reaction (2) is dominant.
The hydrogen generation potential is ignobler than the
equilibrium potential calculated by the Nernst Equation but the excess potential, that is the potential
difference between this equilibrium
potential and the hydrogen generation potential, is affected by
current density, temperature, and the surface condition of the steel. As a result, it is not easy to
determine the excess potential exactly [3].
From the past research, however, it has been confirmed
that the equilibrium
potential can be used as the hydrogen generation potential, disregarding
the
excess potential [4].
Since the pH value of the micro pore solution in sound concrete is generally
about 12.5, the equilibrium potential can be calculated as -934 mVvs Ag/AgCl using the Nernst
Equation.
However, as the pH value around the steel embedded in concrete would increase with
the cathodic reaction of desalination,
the hydrogen generation potential may be a little ignobler than
this level when desalination is applied.
When reaction (2) becomes dominant and hydrogen atoms are generated, these hydrogen atoms
diffuse among rhe crystal lattice of the prestressing steel and accumulate at trapping sites such as
dislocations,
atomic holes, grain boundaries, and inclusion surfaces. Hydrogen lightly trapped at
dislocations,
atomic holes, or grain boundaries is known as diffusible
hydrogen because it is
diffusible
at normal temperatures, and this hydrogen is considered to be the direct cause of delayed
fractures resulting from hydrogen embrittlement [5]. However, there are a number of theories
regarding the mechanism of hydrogen embrittlement,
especially
the way such trace amounts of
hydrogen cause cracking, and the phenomenon remains to be fully clarified.
3.
TESTPROGRAM
The project described here consists of slow strain rate tensile tests using prestressing steel bars
removed from PC prism specimens and flexural tests on PC beams after applying desalination.
-54-
The test program is outlined in Table 1. Each
experimental
factor is examined with two
specimens. In this study, all values of current
density are referenced to the surface of the
prestressing steel bars.
Table 1 Outline of test program
'v n e io i w ;: mg mss mf si f'S asSf tf r e s t r e s s
S p ecim e n
i & S& S S
0
5 0
3.1 Preparing Specimens and Electric
Supply
i K ef W i S S f i Jl i
fiIm i^w a^K ^J L& ^cf ^igJ^jyfim B loB O O J iift ^ H im Hh im r R
i i iZ v B
I K ft K U K K i mi x & & & ｮ m
0 .0
0 .0
8
0 .0
B
0
0 , S d ay s,
ty p e
b) Prism specimen characteristics
The prism specimens were 15x15x40 (cm) with
a prestressing
steel bar at the center of the
square section, and prestressing
was by the
pretensioning
system. In this case, prestress
wasnot introduced to the concrete itself so as to
prevent loss ofprestress due to creep and drying
shrinkage of the concrete, so the prestressing
force reaction was supported by the steel form.
Three types of prestressing steel were used in
the study: two types of PC bars ($13 mm,B-l
type and C-l type) which had been quenched
and tempered by means of high-frequency
heating and deformed cold-drawn PC wire ( </> 9
mm). Inthis paper, C-l type and B-l type PC
bars are represented by C and B, respectively,
and cold-drawn PC wires by CD wire. The
mechanical
properties
and
chemical
compositions
of these prestressing
steels are
shown in Table 3 and Table 4, respectively.
The tension of the prestressing steel was 50% or
60% of its tensile strength.
Furthermore,
prism specimens made with concrete containing
no Cl~ and non-tensioned PC bars were
prepared for reference.
An outline of the
prism specimen is given in Fig. 1.
c) Characteristics
of flexural test PC beams
The test beams had a rectangular cross section
of width lOcmX full depth 20 cm and a total
length of 160 cm. A pretensioned PC bar (B
or C ) was arranged with an effective depth of
13.3 cm in the beam section.
Moreover,
vertical stirrups were provided over the entire
shear span at 10 cm intervals.
Epoxy coated
bars D10 SD295A were used as insulated
stirrups
and erection bars.
The loading
-55-
0
5 .0
Current
a) Mix proportion of concrete
The concrete mix proportion is shown in Table
2. The compressive strength of the concrete
reached 40 MPa at the age of 28 days, which
was the aim in mix proportion
design.
Ordinary Portland cement was used, and refined
salts (99% NaCl) were used as premixed
chlorides.
The amount of Cl~was 8.0 kg/m3,
corresponding
to relatively
severe chloride
attack.
ig jM - K i M t s a j i a
6 0
8
5 .0
7 d ay s,
I m o n th ,
6 m o n th s ,
Iv e ar
0
0 .0
5 0
8
0 .0
0
5 .0
P ri s m
4
0 .0
0
8
c
4
ty p e
0
0 , S d ay s,
6 0
7 d ay s,
5 .0
8
I m o n th ,
6 m o n th s ,
Iv ea r
1 0 .0
8
1 5 .0
C D
6 0
w ir e
0 .0
8
5 .0
8
0
0
0 , 7 d ay s,
1 m o n th
0 .0
5 0
B
0
5 .0
8
0 .0
ty p e
6 0
B eam
0 . 1 m o n th
5 .0
0 .0
5 0
c
0
5 .0
8
0 .0
ty p e
6 0
0 , I m o nth
5 .0
Table 2 Mix proportion of concrete
39
m sfe js i:
43
Table 3
25
p i l lI I I !! n i1i; I JSS am 1 I
1m m KB 1 tape
ra reS8H8H I
hMW
16 9 4 3 4 7 3 1 9 8 2 4 .6 7
8
NMS: Nominal maximumsize of aggregate
Mechanical properties of PC bars
It iii K'IKK H B l
FSf
flflffi MIB
Bj'Jn qsIBI 8E lo ng a tio n
0 *is& &ijO ｣8 m m m i & u iJsiM y 8iJHSflEf
lfSf
M fiaf
i&88a
B^ fiW fi ^8ooBA8fflMw IM8889
tiil
H I
^
i pi H
B ty p e (S B P R 9 3 0/1 08 0 )
10 4 7
11 1 5
1 0 .0
C typ e (S B P R lO 8 0/12 30 )
12 2 8
12 7 3
8 .0
C D w ire (S W P D I L )
14 1 5
16 0 3
6 .5
Table 4 Chemical compositions
B fe w -SB iｻ ?M ｫ
m
of PC bars
EK B iffl B iM i ^ Si'tK gH
S i;;
B ty p e (S B P R 9 3 0/1 08 0 ) 0 .35 1 .74
C typ e (S B P R lO 8 0/12 3 0 ) 0 .35 1 .74
C D w ire (S W P D I L )
0 .82 0 .2 4
m m
0 .7 4 0 .0 16 0 .0 06
0 .7 4 0 .0 16 0 .0 06
0 .7 5 0 .0 12 0 .0 08
0 .0 1
0 .0 1
0 .0 1
condition
of test beams is shown in Fig. 2.
C u r re n t p a s s in g
c o n c re te s u r f a c e
¥
d) Electric current supply procedure
In the case of the prism specimens, concrete was
cast around a tensioned PC bar. Then, after
curing with a wet mat covering for 28 days,
electrodes were placed around the specimen,
which was immersed in electrolyte.
Then
direct current was applied (refer to Fig. 1).
The PC beams were covered with a wet mat
after casting for 28 days.
After curing,
prestress was introduced.
Electrodes were then
placed around the specimens, which were
immersed in electrolyte.
Direct current was
applied through the PC beams. A saturated
solution of Ca(OH)2 was used as the electrolyte
and titanium mesh plated with platinum formed
the anode.
Y
S p e c im e n
E lt c tro lv te lia u id
Prestressing
steel
Steel
^
10 0
r
I
5J
^ W w 19
fram e
Prism type specimen
8 0 0
150
d
C o a tin g
bar
Fig. 1
r n
rF
5 0 0
P /2
Jt
f
1 50
S h e ar
re in fo r c e m
20 0
~i
Unit:mm
en t
13 3
/
' PCbarI100
Fig. 2 Beam type specimen
The standard current density was 5.0 A/m2 and
the standard period of treatment was 8 weeks.
Furthermore, certain specimens were treated with a current density of 10.0 A/m2 or 15.0 A/m2 to
investigate the influence of current density.
The electric current was passed between two sides of
each specimen for the specified period, while the other faces were insulated with an epoxy resin
coating.
Specimens not subjected to electric current were also kept in the electrolyte solution
during the period of treatment.
Specimens used to investigate the influence of elapsed time after finishing treatment were stored
with the prestressing force maintained for the specified period (3 days, 1 week, 1 month, 6 months,
and 1 year). The storage conditions were a constant 20 C, 60% R.H.
Whenthe specified treatment and storage were finished, tests were carried out. Prestressing steel
removed from the specimens was cooled to a temperature of ~30°C to prevent evolution of
absorbed hydrogen from the prestressing steel.
3.2
Slow Strain Rate Tensile Test
Slow strain rate tensile
specimens after finishing
tests were conducted
determined treatment.
on PC bars or PC wires removed from prism
a) Test to evaluate delayed fracture sensitivity
The most serious problem caused by the hydrogen embrittlement of high-strength
steel is the
increase in delayed fracture sensitivity.
Tests to evaluate delayed fracture sensitivity
can be
generally classified
into constant load or strain tests and slow strain rate tests (SSRT) [6].
Constant load tests, as represented by the FIP test, estimate the delayed fracture sensitivity
by the
time taken before cracking or fracture of the steel occurs under a constant load. On the other hand,
slow strain tests estimate the delayed fracture sensitivity
from certain characteristics
of steel
obtained through tensile tests with very small strain rates (about 10"6/sec).
In this research, the electric current was applied to steel embedded in concrete, not to steel
immersed in a solution simulating concrete, and the change in properties of the prestressing steel as
it absorbed hydrogen was investigated.
A slow strain rate tensile test was selected because it
allows for rapid estimation before evolution of the hydrogen.
b) Test procedure
PC bars and wires for test were taken from a freezer immediately
-56-
before the test and frost was
removed from the steel surface with a cloth.
The steel section was not shaved for the test. The
fixed strain rate used in tensile testing was 1.6X10~5/s.
The load was measured by a load cell with
a maximumcapacity of 20 tf, and the strain of PC bars and wires was measured with a plastic strain
gage. The crosshead displacement
of the universal testing machine was measured with a
displacement
meter which had a maximum capacity
of 50 mm (sensitivity:
0.01 mm).
Furthermore, the contraction rate of the fractured steel section was defined by the equation below.
The sectional area of the fractured steel was measured following JIS Z 2241.
<p=(A0 -A)/A0 XWQ (%)
(3)
where
A0 : original area (mm2)
A : minimum area offractured
section (mm2)
3.3
Flexural
Testing of PC Beams
Flexural tests were carried out on PC beamsafter completing the specified
treatment as follows.
a) Measured points
Load and mid-span deflection
were measured by means of a load cell (capacity:
displacement meter (capacity:
25 mm; sensitivity:
0.01 mm), respectively.
10 tf) and
Furthermore, to measure crack width at the main reinforcement level (d=13.3 cm) of the flexurai
span, six 50 mm 70 gages (capacity:
2 mm; sensitivity:
0.001 mm) were inserted without spaces.
b) Loading
Each loading step was 0.25 tf until the yield point, and thereafter,
loading was controlled
by
deflection
at mid span; that is, one step was 0.3 mmwhen the deflection was less than 5 mm, and
beyond 5 mmone step was 0.5 mm. Whena flexurai crack was generated, the beam was unloaded
to 0.25 tfonce and loaded again until the load reached up to 80% of the maximumload.
All 70 gages were removed when the gage indication reached about 2 mm. Moreover, the crack
spacing at the main reinforcement level was measured after finishing the loading test. The loading
conditions for all beams are shown in Fig. 2.
3.4
Measurement of Half-cell
Potential
Changes with time in half-cell potential of the prestressing
steel embedded in the PC beams were
measured. As a reference electrode, a saturated silver chloride electrode (Ag/AgCl)
was used.
HALF-CELL POTETTAL OF PRESTRESSING
STEEL
0
4.
Non-corrosion
0
Changes with time in the half-cell
potential
of
prestressing
steel embedded in the PC beams were
measured for 1 month after 8 weeks of treatment at a
current density of5.0 A/m2. Results for beams using
type C PC bars tensioned to 60% of their tensile
strength are shown in Fig. 3. Each line in Fig. 3
correspond to one specimen, and the four divided
areas (Non-corrosion,
Uncertain, Corrosion, and
Protection)
shown follow the ASTM criterion for
adding cathodic protection area as given by the JCI
0
.£
-0.2
~efo
^.0.4
>
¥
o
-0.6
-0.8
Protec
CL,
à">^á"
JbquiliDnum potential
-1
C type, 60% pretensioned
-1.2
From Fig. 3, the half-cell potentials
specimens were classified
in the
Corrosion area because PC bars were
corrosion environment. On the other
of non-treated
Uncertain or
exposed in the
hand, those of
treated specimens just after finishing
treatment were
-57-
0
5
10
15
20
25
30
Period after Sweeks treatment (days)
Fig. 3 Change in half-cell potential
after 8 weeks of treatment
35
classified
in the Protection area. These potentials
classified
in the Protection area are ignobler than the
hydrogen generation potential because of the strong
influence of the electric field.
As a result, it is
certain
that
hydrogen
was generated
during
desalination
due to the cathodic reaction even if
account is taken of the higher pH value around the PC
bar, and it is considered that hydrogen embrittlement
of PC bar occurred. After stopping the electric
current, PC bars were repassivated with an oxygen
supply and potentials
gradually became more noble.
From Fig. 3, it is seen that half-cell potentials
of
treated specimens became nobler than the hydrogen
generation potential 5 days after desalination ended.
0
5
10
15
20
25
30
35
Displacement of crosshead (mm)
Ishii et al. reported in a study of changes in absorbed
Fig. 4 Relation between steel strain and
hydrogen up to 1 week after stopping the electric
crosshead displacement (Original
bars)
current that the peak of diffusible hydrogen absorption
into PC wire falls with elapsed time and that the effect
of hydrogen embrittlement thus tended to be relieved [5].
At this stage, it is not clear whether
there is a relation between the change in half-cell potential after desalination
and the relief of
hydrogen embrittlenent or not. However, it can be said that the risk of delayed fracture due to
hydrogen embrittlement
decreases rapidly if the half-cell potential
remaines nobler than the
hydrogen generation potential.
5. SLOWSTRAIN RATETENSILE TESTS ON PRESTRESSTNG STEEL
5.1
Estimation
of Stress-Strain
Relation
In this study, the approximate stress-strain
curve until the fracture point was obtained by
measureing tension load, tendon strain, and crosshead displacement of the universal testing machine
during slow strain rate tensile tests.
Generally, the elongation behavior of a steel bar can be divided into two parts. First is the overall
elongation that occurs until near the maximumstrength point, and second is the local elongation
from close to the maximumstrength point until the fracture point.
Plastic strain gages were used
to measure the overall elongation part. However, when local elongation developed in an area not
covered by the strain gage, it was impossible to continue measurement of elongation until fracture.
The relations between prestressing steel strain and displacement of the crosshead in the case of the
original (as-manufactured)
bars and wires are shown in Fig. 4. From this, it can be said that
although displacement of the crosshead results in a large relative steel strain in the elastic and early
plastic stages because of the reduction in steel bar section, there is a linear relation in the plastic
stage of 3% to 5% of steel strain.
Beyond this stage, the linear relation is lost due to the
dominance of local elongation.
In this study, the steel strain after 5% was calculated
approximately
by means of the linear relation between steel strain and displacement
of the
crosshead in the 3% to 5% range, and a rough stress-strain curve until the fracture point was
obtained.
Although each steel bar or wire had its own approximate equation, the range of data
used in the approximation was constant (3% to 5% steel strain).
5.2
Hydrogen Embrittlement
Behavior of Certain Prestressing
Steel
Stress-strain curves of specimens treated for 8 weeks and tensioned until 60% of tensile strength
are shown in Fig. 5. The influence of treatment on the contraction rate of fractured steel section is
shown in Fig. 6. In these figures, data for the original steel are marked as 'Original'
for
comparison. From Fig. 5, the influence of desalination
on elastic behavior and plastic behavior
-58-
1800
70
o
60% pretensioned
1600
A
1400
50
1200
800
£
600
400
D CD-Original,
O C-Original,
A B-Orignaj
CD-Treated
C-Treated
B-Treated
i:
20
0
10
10
of treatment on stress-strain
C D w ire
c typ e
B ty p e
A
30
g7S
E3¥
12
-Ocd
a>
Tc4>^3
-Oc4>^
A
o
T^>
c^3
o
Strain (%)
Fig. 5 Influence
O
A
o
6
200
o
A
40
s 1000
I
o
2 60
relation
before the load begins to decreasing is seen to be
very little.
From this, it can be said that there is
almost no influence of desalination
on such Fig. 6 Influence of treatment on
characteristics
of prestressing
steel as elastic
contraction rate of fractured section
coefficient,
yield strength, yield strain, and tensile
strength.
On the other hand, the influence ofdesalination
is clearly visible once the load begins to
decrease and up to the fracture point. As a consequence, the ultimate state characteristics,
such as
strain when the load begins to fall, the length of the load-decreasing zone and contraction ratio of
fractured steel section change. As already noted, steel strains in the ultimate state are calculated
approximately
and it is difficult
to estimate the absolute values themselves.
However, it is
possible to compare them relatively.
In both type of B and C cases, the strain of treated specimens
whenthe load begins to fall is a little less than that of the original ones. On the other hand, in the
case of CD wire, the strain when the load begins to fall is reduced significantly
by treatment.
Furthermore, the length of the load-decreasing
zone is reduced by treatment for all types of
prestressing steel.
From Fig. 6, the contraction rate of the fractured steel section decreases with treatment for all types
of prestressing steel and this corresponds to brittle fracturing of the treated specimens as shown in
the stress-strain curves. It is deduced that these phenomena are caused by hydrogen embrittlement
of the prestressing steel due to the application of desalination,
and when the difference in behavior
between treated specimens and original
ones is considerable,
the influence of hydrogen
embrittlement
and the sensitivity
of delayed fracturing is great. Treatment periods for type C
specimens were 4 weeks and 8 weeks. The longer treatment results in a greater decrease in
contraction rate of fractured steel section, which would correspond to a greater degree of hydrogen
embrittlement.
A difference in prestressing
force (between 50% and 60% of tensile strength) does not cause a
significant
change in tension behavior. However, Suzuki et al. reported that when plastic strain
wasintroduced to steel, the absorption of diffusible hydrogen increased because of the generation of
trapping sites [8]. The prestressing forces in this study were determined from the permissible
tensile stress of prestressing steel under the design load as regulated in the Specifications
for Design
and Construction of Road Bridges, are premised on the elastic behavior of prestressing
steel.
However, when desalination is applied to structures that experience very heavy loading, it is
possible that the delayed fracture sensitivity becomes high.
The relation between the ratio of treated specimen contraction rate to original one and tensile
strength is shown in Fig. 7. The data shown in Fig. 7 were obtained immediately after 8 weeks of
treatment and 7 days later. Comparing the delayed fracture sensitivity
of heat-treated steel and
cold-worked steel, it is generally accepted that cold-worked steel is less sensitive to delayed fracture
-59-
1800
than heat-treated steel for the following reasons [9]:
à"The dense dislocations
introduced
by coldworking become effective
trapping
sites
for
hydrogen.
à"The fiber structure of steel introduced
by cold
working resist the development of cracks in the
fracture direction.
^r- 140°
1 1200
5 1000
_
Just after treatment
I
800
§
600
400
J
7days after treatment
200
0
0
0.2
0.4
0.6
0.8
1
Ratio of contraction rate to original one
Fig.
7
Relation between contraction rate and
tensile strength
1.0,
0.9
o
PC Wire
2
0.8
CO
in
0)
0.7
PCBar
w
On the other hand, if the decrease in contraction
ratio means increased delayed fracture sensitivity,
Fig. 7 shows that the delayed fracture sensitivity
of
cold-worked steel is higher than that of heat-treated
steel. The reason for this is considered to be that
as the heat-treated
steel used in this study was
quenched and tempered for a short time by highfrequency heating, the binding
force of grain
boundaries is greater and the delayed fracture
sensitivity
is lower than that of ordinary tempered
martensite steel for the following reasons [10]:
à"Thecrystal grains are fine.
à"Film-type cementites are not extracted at the grain
boundaries.
à"The amount of extracted carbide is little and the
extracted carbides are fine.
1600
Moreover, somekinds of steel used in this research
have different tensile strengths and as shown in Fig.
7, it can be said as a universal interpretation
independent of the steel manufacturing process that
the higher strength steel exhibits the higher delayed
fracture sensitivity.
TJ
O.E
D.
Q.
<
1
10
100
1000
Timetofracture
(h)
Takai et al. conducted FIP tests on PC bars and
Fig. 8 Relationship
between applied stress ratio
wires like the prestressing steel used in this study,
and the relation between applied stress ratio
and time to fracture of PC wire and PC bar
in FIP test9)
(applied stress/tensile
strength) and time to fracture
wasas shown in Fig. 8 [9]. This indicates that PC
wires has better delayed fracture resistance than PC bars as is generally accepted because the limit
of applied stress ratio to fracture is larger for PC wire than for PC bars. However, the tune to
fracture of PC bars is generally longer than that of PC wire, as seen in Fig. 8. Since the slow strain
rate tensile tests used in this study result in steel fracture, there are some important differences from
constant load tests like FIP tests and it is possible that time to fracture shown in Fig. 8 is
emphasized in slow strain rate tensile tests. Considering this, it is important to evaluate the
delayed fracture characteristics
of steel fromvarious points of view in the future.
5.3
Influence
ofPre-mixed
Cl~
The main subject of this study is hydrogen absorption caused by electric current treatment, but it
has been reported that corrosion reactions can generate hydrogen and that this hydrogen may be a
cause of delayed fracture in high-strength
steel [11].
In the case of this study, the steel bars removed fromtreated specimens looked sound and did not
appear to be corroded, but bars removed fromnon-treated specimens were covered with corrosion
products.
Figure 6 shows that compared with original specimens, non-treated specimens show
little decrease of contraction rate of the fractured steel section.
Stress-strain
curves and
contraction rate of the fractured steel section in the case of non-prestressed and non-treated
specimens are shown in Fig. 9 and Fig. 10, respectively.
In one case, concrete with added chloride
-60-
1OUU
1400
cS
a.
^ /u
0
.9 60
CJ
1>
50
<o
3
3 40
US
tfa
30
D
i: 20
o
ｧ 10
l_
e
o n
0 % pretensioned,
non- treated
1600
1200
¥ -
1000
V0-
800
600
400
D
O
200
A
n
C -O riginal,
c -W ithoutC L
C -W ith C L
I
2
A
I
4
B -O riginal
B -W ithoutC
I
6
B -W ith C l
I
8
I
10
Fig. 9
DBCttyyppeeI
Original
12
Strain(%)
I n f l u e n c e o f C l ~o n s t r e s s - s t r a i nr e l a t i o n
5 . 3 I n f l u e n c eo f H i g h C u r r e n Dt e n s i t y
WithoutCl"
WithCl"
Fig. 10
I n f l u e n c oe f C l o n c o n t r a c t i o n
r a t e o f f r a c t u r e ds e c t i o n
( 0 % p r e t e n s i o n e d ,n o n - t r e a t e d )
w a su s e da n di n t h e o t h e r ,n o c h l o r i d e s w e r ea d d e d .
F r o mth e s e f i g u r e s , it is c l e a r that s p e c i m e n sw i t h o u t
Cl~ s h o w the s a m e behavior as the original
s p e c i m e n s ,t h o u g h in t h e c a s e o f s p e c i m e n sw i t h C F
t h e e a r l y o n s e t o f l o a d r e d u c t i o n a n d a d e c r e a s ei n t h e
c o n t r a c t i o nr a t e a r e i n d i c a t e d .
^ /u
c
.2 6 0
o
<u
50
4>
¥-3
6 40
Kt
tb
30
1)
20
o
a lO
o
F r o mt h e a b o v e ,it a p p e a r s p o s s i b l e that prestressing
steel in a c o r r o s i v e e n v i r o n m e natb s o r b s h y d r o g e n
g e n e r a t e db y t h e c a t h o d e r e a c t i o n a n d this hydrogen
causes delayed fracture due to hydrogen
e m b r i t t l e m e n t , e v e n if p r e s t r e s s i n g steel is not
influenced by a n electric current. T h e r e m a yb e
s o m ei n f l u e n c e o f s t r e s s c o r r o s i o nc a u s e d b y t h e
a n o d i cc o r r o s i o nr e a c t i o n ,b u t a t p r e s e n t it is difficult
t o divide t h e s e i n f l u e n c e sc l e a r l y .
D
n
o
oo
n
0
p
C ty p e , 6 0 % p re ten sio n e d
n
Original 5 A / m l O A / m 1 5 A / m 2
C u r r e dn et n s i t y
Fig.ll
I n f l u e n coef c u r r e nd te n s i t yo n
c o n t r a c t iroant e
F o rs p e c i m e nu s i n gt y p e C s t e e lt e n s i o n e dt o 6 0 % o f
t e n s i l e s t r e n g t h ,t h e r e l a t i o n b e t w e et nh e m a g n i t u d oe f c u r r e ndt e n s i t ya n d t h e c o n t r a c t i orna t i o o f
f r a c t u r e sdt e e l s e c t i o ni s s h o w inn F i g . l l . A s t h e t r e a t m e np te r i o dw a s8 w e e k isn a l l c a s e s ,t h e
h i g h e r c u r r e ndt e n s i t y c o r r e s p o n tdos t h e p a s s a g eo f a l a r g e re l e c t r i cc h a r g e s, o t h e a m o u no tf
h y d r o g e gn e n e r a t ebdy t h e c a t h o d i cr e a c t i o ns h o u l di n c r e a s ien p r o p o r t i o tno t h e c u r r e ndt e n s i t y .
H o w e vfer ro,mF i g .l l , t h e d e c r e a si en c o n t r a c t i or na t e i n t h e c a s eo f s p e c i m e nt rse a t e da t a c u r r e n t
d e n s i t yo f 1 0 . 0 a n d1 5 . 0 A / m 2is l e s s t h a n t h a t o f t h e s p e c i m et nr e a t e da t a c u r r e ndt e n s i t yo f 5 . 0
A / m 2 .T h i s i n d i c a t e st h a t a t s u c hh i g h c u r r e ndt e n s i t i e s ,t h e a m o u no tf h y d r o g e anb s o r b e bd y t h e
p r e s t r e s s i nsgt e e l i s r e d u c e da n dt h e d e g r e eo f h y d r o g e enm b r i t t l e m e ins tr e l i e v e d .
B a t e i e t a l . r e p o r t e dt h e g e n e r a t i o on f c o n c r e t ce r a c k sw h e na n e l e c t r i cc u r r e nwt a sa p p l i e d t o
p r e t e n s i o n - t y Pp Ce s p e c i m e ni nse x c e s os f p r o t e c t i o rn e q u i r e m e n[ t1 2s ] . A s t h e c a u s eo f t h i s , t h e
e x p a n s i po rn e s s u roef h y d r o g e gn a sg e n e r a t eadr o u nt dh e s t e e l w a s u g g e s t e [d1 2 ] .
A l t h o u g hn o c r a c kws e r oe b s e r v ei dn t h e c o n c r e tseu r f a c de u r i n gt h i s s t u d y e v e nw h e na c u r r e n t
d e n s i t yo f 1 0 . 0 o r 1 5 . 0A / m w
2 a asp p l i e d ,v e r yf i n ec r a c k sm a yb e g e n e r a t eadr o u nt dh e p r e s t r e s s i n g
s t e e l b e c a u s oe f t h e h i g h c u r r e n dt e n s i t y . U n d e rt h e s ec i r c u m s t a n cteh se , a m o u no tf h y d r o g e n
a b s o r b e bd y t h e p r e s t r e s s i n sgt e e l a n d t h e d e g r e eo f h y d r o g e en m b r i t t l e m e na tr el i k e l y d e c r e a s e d
b e c a u st eh e h y d r o g epnr e s s u ri se d e c r e a s et dh r o u g hth e g e n e r a t i oonf s u c hf i n ec r a c k s .
-61-
<L>0.19
0000..87
0S)00..65
.2200..43
50.2
oO
o .1n
1800
60% pretensioned
1600
1400
'0i
1200
s 1000
¥ 800
&
600
400
200
p CD-Just after,à" CD-Imonthaftei]
D C-Justafter, à" C-lmonthafter
L^ B-Just after, A B-lmonthafter
6
0
10
12
Influence of time after desalination
stress-strain relation
on
Strain (%)
Fig. 12
5.4
Relief of Hydrogen Embrittlement
Fig.
m E=lV i
C D w ire
O c type
A B type
Justafter 3 days 7days 30days 6months lyear
Period after finishing Sweeks treatment
13 Influence of time after desalination
on
contraction rate
after Treatment
Suzuki et al. reported a decrease in the amount of diffusible hydrogen absorbed into steel if the steel
is kept at room temperature after charging with hydrogen [8].
Since desalination
is only a
temporary treatment and not a permanent arrangement like cathodic protection, relief of hydrogen
embrittlement is expected.
Changes hi the stress-strain
relation and contraction rate with elapsed time after completion of
desalination
are shown in Fig. 12 and Fig. 13, respectively.
The treatment period was 8 weeks and
the prestressing
force was 60% of the tensile strength.
From Fig. 12, the length of the load
decreasing zone, which was reduced under desalination
returns to a higher value after 1 month.
Moreover, in the case of cold-worked steel, the initial strain at load decrease also rises and as a
result the brittle fracture behavior is unproved. From Fig. 13, the decrease in contraction rate due
to hydrogen embrittlement is relieved with the elapse of time after treatment, regardless of the kind
of steel.
Thus the contraction rates of heat-treated steels recover to the same level as those of
original specimens.
However, the contraction rate of a type C specimen kept for 6 months is smaller than expected.
With this specimen, corrosion pits that would have formed during curing were observed, and the
fracture originated from this corrosion pitting.
It is considered that this brittle fracture was caused
by a stress concentration at the pitting and not by hydrogen embrittlement due to desalination.
Whendesalination is applied to actual structures, there may be corroded prestressing steel present in
the structure.
In such cases, it is possible that the delayed fracture sensitivity
would increase
above that of non-corroded prestressing steel, so adequate investigation
of this point will be needed
hi the future.
fi. FT.EXURALTESTING OF PC BEAMS
Flexural loading tests were carried out immediately after desalination and were completed within 2
days of the end of treatment.
After completing desalination,
it was not possible to freeze PC
beams, so there would have been some evolution of absorbed hydrogen into the prestressing steel.
Furthermore, someof the PC beams were tested after 1 month in the room condition.
In the ultimate state, all specimens indicated flexural failure resulting from crushing of the upper
concrete. Although it is possible
that the prestressing
steel might fracture due to hydrogen
embrittlement, no such phenomenonwas observed in this experiment.
-62-
Table 5 Results offlexural test ofPCbeams
m m ? 0 $ !$ m
^
e s tifi S Sia g s
H^ ^ C Jii fi e ij t ^ ;:
m e & a is m
p e ri o d
5
0
0
0
5
0
0
0
5
0
0
0
5
1 m o n th
0
1 m o n th
5
1 m o n th
0
I m o n th
fl e x iiia ^ e f a e k l iig :
M a x im u m l o a d
5 1 .5
5 6 .2
5 2 .7
8 1.1
8 2 .3
8 0 .1
12 4
15 0
12 8
ty p e
5
4
4
4
4
4
1
6
5
4
4
6
.7
.6
.3
.1
.9
.6
4
4
4
4
5
5
6
0
7
0
.0
.6
.8
.0
.8
75
74
77
72
76
75
76
.5
.5
.8
.9
.5
.4
.3
74
75
78
80
.1
.0
.8
.9
12 5
12 5
190
1 65
2 02
173
188
1 84
5
5
5
4
2
4
1
4
.7
.6
.5
.8
78
72
77
72
68
.9
.5
.4
.4
.2
50
B
ty p e
c
ty p e
B
60
60
60
ty p e
The results for all specimens subjected
flexural loading test are shown in Table 5.
6.1
Load-deflection
w & ti M
M
id M
s iJ & ft ifi i6 S ｫ ｣ ; :
g iS ia s ^
60
c
W
4 5 .7
4 5 .3
2
2
3
2
2
2
2
2
2
2
2
2
2
2
3
2
191
188
165
2
2
1
1
**
6 6 .1
gS ;
2
3
29
02
51
77
***
1
2
180
to the
Relation
Load-deflection
relations at the mid span of 60%
pretensioned specimens are shown in Fig. 14.
From Table 5 and Fig. 14, it is clear that treatment
has a rather the positive
influence,
slightly
increasing flexural rigidity
and maximumload as
oppose to a negative influence of treatment such as
reducing maximumload or increasing deflection.
Moreover, the load at which flexural cracking of
treated specimens occurs is generally larger than
that of non-treated ones. Ishii et al. conducted a
static loading test on PC beams with cathodic
protection at a maximum current density of 40
mA/m2to the steel surface for 3 years [13].
From
this, it was observed that the deflection of the
treated specimens was less than that of non-treated
ones and there was no notable influence of
treatment on the load at which flexural cracking
occurred and the maximum load. These results
obtained by Ishii et al. generally accord with the
results obtained in this study.
Ishii et al. concluded that there was no bond
degradation
as a result of treatment because
concrete cracks were not observed along the
prestressing steel [13].
However, considering the
results of the slow strain rate tensile tests on the
prestressing steel itself, which indicated no change
-63-
2
4
6
8
10
12
Deflection at mid span(mm)
8
10
12
Deflection at mid span(mm)
Fig. 14
Load-deflection
relation
at mid span
in elastic rigidity or strength due to treatment, it is
deduced that desalination
does not significantly
alter the properties of the prestressing steel itself,
and the change in bond interface between concrete
and steel influences the mechanical behavior of
treated
PC beams.
Furthermore, from
observations of prestressing steel removed from PC
beams after flexural loading tests, it was found that
corrosion of prestressing steel was slight and it was
difficult
to imagine a decrease in flexural strength
of non-treated PC beams due to corrosion of the
prestresing steel.
6.2
90
80
70
^60
I50
140
_i
30
20
10
C type, 60% pretensioned
0.5
Cracking Behavior
1
1.5
Crack width(mm)
From the data on average crack spacing and the
number of cracks, as given in Table 5, the
influence of desalination
on the crack distribution
is not clear. However, it is not considered that
treatment has a negative influence on crack
distribution
and a small improvement due to
treatment is noted.
The road-average crack width relations
of the
specimens mentioned in Fig. 14 are shown in Fig.
15. The average crack width is the average value
for the measured data obtained using the attached
T0gages across the cracks at the flexural span of
the PC beam. These results show that the crack
width of treated specimens is generally smaller
than that of non-treated
specimens under
equivalent loading conditions.
Thus, since the
crack development rate of treated specimens is
slower than that of non-treated ones, the deflection
of treated specimens is a little less than that of nontreated specimens.
90
80
70
^60
I, 50
§40
J30
20
10
B type, 60% pretensioned
0.5
1
1.5
Crack width(mm)
Fig. 15
Load-average crack width relation
The authors have applied desalination
to RC specimens and reported a decrease in bond strength
due to the softening of cement paste around the reinforcing steel [14].
Generally, when bond
strength decreases, crack width increases because the crack distribution
deteriorates.
However, as
the concrete used in this experiment was relatively high strength compared with RC specimens, the
decrease in bond strength may not have been significant.
As a result, the crack distribution
got no
worse and it was possible that the crack development slowed because of the softening of the cement
paste around the prestressing steel.
6.3
Possibility
of Applying
Desalination
to PC Structures
The results of flexural loading tests on PC beams indicated that there was no negative influence of
desalination on the mechanical behavior of PC beams. However,the main problem of hydrogen
embrittlement of prestressing steel is the delayed fracture phenomenon, and this experiment does
not cover this point adequately enough. Moreover, it is possible that the prestressing steel may be
severely corroded when desalination is applied to actual structures.
Thus, more investigations
including a study of the correlation between hydrogen embrittlement cracking and stress corrosion
cracking is needed in the future.
-64-
7.
CONCLUSIONS
The results obtained
in this project
can be summarized as follows.
(1) Half-cell potentials of prestressing
steel embedded in PC beams treated with desalination
for 8
weeks at a current density of 5.0 A/m2 were ignobler than the hydrogen generation potential just
after finishing treatment. This finding confirmed that hydrogen generation occurred as a result of
the cathodic reaction during treatment. However, the half-cell potentials
of prestressing
steel
became nobler than the hydrogen generation potential when the specimens were kept in room
condition for 5 days after desalination.
(2) Slow strain rate tensile tests on prestressing steel removed fromtreated PC prism specimens
indicated no change in elastic behavior nor strength-related
properties but the fracture behavior was
altered by the absorbed hydrogen.
For example, the load decreasing zone in the stress-strain
curves was shortened and the contraction rate of the fractured steel section decreased as compared
with non-treated specimens.
(3) As a result of an investigation
of the influence of prestressing
force of prestressing
steel,
prestressing forces between 50% and 60% of the tensile strength of each steel type were found to be
insignificant.
(4) As a result of comparing the
of the cold-worked PC wires,
embrittlement
behavior in this
consideration the tensile strength
hydrogen embrittlement
behavior of heat-treated PC bars with that
the cold-worked steel was found to have severer hydrogen
study. Such a tendency can be interpreted
universally
in the
of steel.
(5) Prestressing steel embedded in specimens pre-mixed with Cl~ showed enhanced brittle fracture
behavior in the slow strain rate tensile tests. From this, it was confirmed that prestressing
steel
under a corrosive environment present the risk of delayed fracture even if there is no influence of
hydrogen embrittlement due to electric current.
(6) As a result of an investigation
of the influence of current density applied during desalination,
worse hydrogen embrittlement behavior was observed in specimens treated at a current density of
5.0 A/nr than at a density
of 10.0 or 15.0 A/m2.
(7) The hydrogen embrittlement of prestressing steels was relieved with the elapse of time when
specimens werekept in room condition.
From this, it was deduced that the risk of delayed fracture
due to hydrogen embrittlement caused by desalination
was rapidly reduced, but the risk of delayed
fracture originating from corrosion pitting generated before treatment remained.
(8) As a result of flexural loading tests on treated PC beams, brittle failures such as the fracture of
prestressing
steel due to hydrogen embrittlement
were not observed, and both loading and
deflection performance was not reduced by the treatment, compared with the non-treated
specimens.
(9) The average crack width of treated PC beams was a little smaller than that of non-treated
specimens for the same loading conditions.
As a result, the deflection of treated specimens was a
little smaller than that of non-treated specimens.
Acknowledgements
The authors wish to express their sincere gratitude to Mr. N. Tanaka of Neturen Co. Ltd. for his
cooperation and helpful advice and to Ms. M.Nishio, Mr. H. Yamaguchi, and Mr.T. Yamashita for
their assistance with experiments conducted at the University of Tokushima.
Part of this research was supported by Grant-in-Aid for the Encouragement of Young Scientists
fromthe Ministry of Education (Grant No. 09750544).
-65-
References
[I]
[2]
[3]
[4]
[5]
[6]
Ueda, T., Ogawa, T., Miyagawa, T., and Ashida, M., "Corrosion Behavior of Reinforcing
Steel
after Applying
Desalination,"
Proc. of the Japan Concrete Institute,
Vol. 18, No. 1, pp.11011106, 1996 (in Japanese)
Bennett, J. E. and Schue, T. J., "Electrochemical
Chloride
Removal from Concrete: A SHRP
Contract Statues Report," Corrosion'90,
Paper Number 316, 1990
Japan Concrete Institute,
"Report of Research Committee on Cathodic Protection
for Concrete
Structures,"
1994 (in Japanese)
Ishii, K., Seki, H., and Fukute, T., "Hydrogen Embrittlement
of Prestressing
Steel," Journal of
Materials, Concrete Structures and Pavement, No. 532/V-30, pp. 131-140, 1996 (in Japanese)
Suzuki, N., Ishii, N., Miyagawa, T., and Harada, H., "Estimation
of Delayed Fracture Property
ofSteels,"
Iron and Steel, Vol. 79, No. 2, pp. 227-232, 1993 (in Japanese)
Suehiro,
K., Yamashita, E., Mizoguchi,
S., Tanimura, M., and Shimada, T., "Examination of
Delayed
Fracture
Tests on Prestressing
Vol. 32, No. 353, pp.94-100,
[7]
[8]
[9]
[II]
[12]
[13]
[14]
Steels,"
(in Japanese)
Journal
of the Society
of Materials
Science,
Japan Concrete Institute,
"Guideline
for Protection
of Marine Concrete Structures (draft),"
1990 (in Japanese)
Suzuki, N., Ishii, N., and Tsuchida, Y., "Diffusible
Hydrogen Behavior in Pre-strained
High
Strength Steel," Iron and Steel, Vol. 80, No. ll, pp. 855-859, 1994 (in Japanese)
Takai, K., Seki, J., and Homma,Y, "Hydrogen Occlusion Behavior during Delayed Fracture in
Cold Drawn Steel Wire and Heat Treated Steel Bars for Prestressed
Concrete," Iron and Steel,
Vol. 81, No. 10, pp. 1025-1030,
[10]
1983
The Iron and Steel Institute
Japanese)
1995
(in Japanese)
of Japan, "Advance of Research on Delayed Fracture," 1997 (in
Ikeno, K., Nishimura, R., and Yamakawa, K., "Hydrogen Absorption
of Carbon Steel under
Corrosion Atmosphere," Proc. of the 42th Conference on Corrosion and Protection,
pp.565568, 1995 (in Japanese)
Batei, S. and Takewaka, K., "Application
of Cathodic
Protection
for Prestressed
Concrete
Structure,"
Proc. of the 46th Annual Conference of the Japan Society of Civil Engineers 5,
pp.360-361,
1991 (in Japanese)
Ishii, K., Seki, H., Fukute, T, and Ikawa, K., "Experimental
Study on Pretension PC Beam
Given Cathodic Protection,"
Proc. of the 22th JUCC Congress on Cement and Concrete,
pp.91-96, 1995 (in Japanese)
Ueda, T., Hattori, A., Ashida, M., and Miyagawa, T, "Influence of Desalination
on Bond
Behavior between Concrete and Reinforcing
Steel," Journal of Materials, Concrete Structures
and Pavement, No. 550/V-33, pp. 53-62, 1996 (in Japanese)
-66